1. Field of the Invention
The invention herein relates to devices for dispensing of liquids, typically medications, fragrance-producing liquids or liquids producing desired vapors, in a controlled manner. More particularly it relates to portable, self-contained liquid dispensing devices which can dispense minute controlled quantities of liquids over a prolonged period.
2. Description of the Prior Art
In recent years there has been a substantial growth in the field of devices which are intended to dispense small amounts of liquids over an extended period of time at closely controlled low flow rates. While such devices have found uses in many field, they have been particularly valuable in the field of medicine and pharmaceuticals. In-patient treatment methods which rely on medications administered by such devices have proved quite successful for many diseases and dysfunctions. (For brevity, in much of the following discussion, the prior art devices and the devices of this invention will be exemplified for use in liquid medication dispensing. It is to be remembered, however, that the devices are usually equally functional for dispensing of other liquids, as will also be described below.) Many medical treatment protocols require slow administration of medications over prolonged periods, such as days or weeks, during which time it is undesirable to require a patient to remain confined to a medical facility in order to be properly medicated. Therefore small self-contained portable delivery devices that the patient can have implanted, carry or wear while going about his or her normal daily routine have been developed for use in such protocols. These devices are a necessity for successful out-patient treatment, treatment of long-term chronic conditions or in the convalescent stage of treatment following trauma, surgery or acute illness. These devices provide slow administration of medications over an extended time in the patient's home or otherwise away from a medical facility, in a setting in which the patient can be unrestricted in his or her movements.
There have been a number of different kinds of self-contained dispensing devices put into the marketplace. All require three components for successful operation. There must be a reservoir for the medication, a transfer component such as a medication-permeable film attached to the reservoir to serve as a conduit for the medication from the reservoir into the patient's body, and some sort of motivating force to cause the medication to move from the reservoir through the transfer component into the patient. The most common transfer component is a needle or cannula used to inject medication subcutaneously into the patient's veins, arteries, muscle or tissue. However, most recent research and development work has focused on the use of less invasive transfer components such as patches for transdermal administration, which require new types of motivating devices.
Various kinds of motivating devices have been described. There are passive pumping technologies such as osmotic devices and controlled or sustained release devices. The osmotic devices require a source of aqueous fluid such as water or aqueous body fluids. The motivating fluid must be in at least as great a volume as the fluid to be dispensed, since the devices work by fluid exchange. The controlled release devices are based on diffusional pressures through membrane barriers and require special formulations for the medication and the membranes in order to function properly.
Gas pressure devices for moving fluid have also been developed. Most have used gas evolution from a gas generating material to apply pressure to a flexible gas/liquid barrier, such as a rubber sheet. The expanding gas then presses against the flexible barrier and forces the liquid out of the adjacent liquid chamber and through the administration component. Gas generation in these kinds of devices commonly is obtained by reacting two chemicals together with one of the reaction products being the desired gas, or by dissolving a gas-containing material in a solvent such as water to cause evolution of the gas. There are severe limitations on these types of devices, however, since there are only a limited number of chemicals which when reacted in small quantities will generate sufficient gas to be a driving force in a dispensing device, or which can be dissolved to form such a quantity of gas. Moreover, such devices are necessarily further limited as to the gas itself which may be used and to the potential chemical reactants for production of the gas. The devices must involved use only those gases and gas generating chemicals which are safe, non-toxic and non-allergenic for the patient, which can be easily and properly handled and which are compatible with the other components of the device, particularly the gas/liquid barrier.
Further, and most importantly, these devices are limited in the time period over which they will operate, since gas generation or evolution is a relatively rapid phenomenon and ceases as soon as the reactant chemicals are fully reacted or the soluble material is fully dissolved. Thus, while the gas pressure will be high early on, as the device continues in use over a period of time the gas pressure will fall and the ability of the device to dispense the medication fully at an accurate and desired flowrate will diminish. Such devices therefore find their greatest use in treatment regimens where the medication in the device is to be administered to the patient over a relatively short time, such as a few hours or days.
Another type of gas generating device has much greater flexibility and ability to be used for sustained administration of medications. These are the devices that depend upon electrochemical pumps to move the liquid through the dispensing device and into the patient. I have previously developed a number of electrochemical pumps as well as the drug dispensing devices which utilize such pumps as the motivating force. A particularly relevant previous device which I developed is illustrated in my U.S. Pat. No. 4,902,278 entitled "Fluid Delivery Micropump" (issued in 1990). This patent describes a electrochemical prime mover module for use in fluid delivery micropump (especially for pharmaceutical purposes) which uses an ion exchange cell in which water is reduced and pure oxygen regenerated to apply external gas pressure to a collapsible reservoir and expel a fluid from the reservoir. (As will be evident from the description below, the prime mover described in that patent is also an important element in the present device.)
Another prior art device relevant to the present invention is described in U.S. Pat. No. 4,734,092 to Millerd (issued in 1988). Millerd describes a device to inject a drug into the human body with a needle via intravenous, intraarterial, intramuscular and subcutaneous routes.
Some of the prior devices, including mine, have been commercially successful in various settings. However, no one has yet been able to provide an effective micropump with a direct gas-liquid interface in which the interfacial surface area is sufficiently small to reduce problems such as gas dissolution in the liquid and interphase contamination to an insignificant level, while simultaneously being able to steadily and accurately transfer the fluid to a porous surface from which it can be removed by evaporation or diffusion.